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Duration of volunteer potato (Solanum tuberosum) interference in bulb onionAuthor(s): Martin M. Williams II, Corey V. Ransom, W. Mack ThompsonSource: Weed Science, 53(1):62-68. 2005.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/WS-04-079R1URL: http://www.bioone.org/doi/full/10.1614/WS-04-079R1
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62 • Weed Science 53, January–February 2005
Weed Science, 53:62–68. 2005
Duration of volunteer potato (Solanum tuberosum)interference in bulb onion
Martin M. Williams, IICorresponding author. United States Department ofAgriculture–Agricultural Research Service, InvasiveWeed Management Research, University of Illinois,N-325 Turner Hall, 1102 South Goodwin Avenue,Urbana, IL 61801; [email protected]
Corey V. RansomMalheur Experiment Station, Oregon StateUniversity, 595 Onion Avenue, Ontario, OR 97914
W. Mack ThompsonPioneer Hi-bred International Inc., P.O. Box 552,Johnston, IA 50131
Previous research with annual weed species indicates that critical timing of weedremoval begins primarily after the two-leaf stage of onion, a time when postemer-gence (POST) herbicides can first be applied. Volunteer potato is difficult to manageand persists in onion fields of western United States. The purpose of this researchwas to quantify the duration of volunteer potato interference on yield and marketgrade of onion as well as potato tuber production. Volunteer potato interferencecaused a 5% yield loss before onions reached the two-leaf stage, at two of threelocations. Relative to weed-free plots, onion bulb diameter was reduced as durationof interference increased, resulting in smaller proportions of marketable bulbs. Vol-unteer potato produced daughter tubers shortly after emergence, which explains, inpart, weed persistence despite removal of shoots with contact herbicides, cultivation,and hand-weeding in onion. Significant losses in onion yield and bulb diameter arelikely given current volunteer potato management systems.
Nomenclature: Volunteer potato, Solanum tuberosum L. ‘Russet Burbank’, ‘Rang-er Russet’; onion, Allium cepa L. ‘Pinnacle’, ‘Vaquero’.
Key words: Competition, critical time of weed removal, critical weed-free period.
Commercial potato harvest leaves numerous tubers in thefield, which are distributed throughout postharvest tillagedepths, and volunteer potatoes arise from these nonharvest-ed tubers (Lumkes 1974; Perombelon 1975). In several po-tato-producing regions of the world, winter temperaturesfreeze tubers near the soil surface; yet, tubers deeper in thesoil profile survive and emerge in rotation crops (Lutman1977; R. Boydston, unpublished data). Volunteer potatoesare difficult to suppress because of large energy reserves inthe tuber and the relatively deep burial depth comparedwith annual weed species. Short periods of vegetative growthmay contribute to volunteer potato persistence because ini-tiation of new tubers begins shortly after emergence. Vol-unteer potato persistence reduces the benefits of crop rota-tion because the weed can serve as an alternate host andsource of inoculum for serious disease, insect, and nematodepests of potato (Ellis 1992; Thomas 1983).
Bulb onion, hereafter called ‘‘onion,’’ is an important cropin western United States, where it is often grown in a 3-yrrotation with potato. Onions are susceptible to weed inter-ference because the crop is slow to emerge, has a low initialgrowth rate, and its narrow, erect leaves produce little shade(Hewson and Roberts 1973; Wicks et al. 1973). Volunteerpotato interference can result in 85% total yield loss (Boyd-ston and Seymour 2002), with complete loss of the mostvaluable market grades (Williams et al. 2004a).
Onion weed management systems rely heavily on post-emergence (POST) tactics because volunteer potatoes arenot controlled by preemergence herbicides registered for usein onion. Weed management strategies for control ofemerged volunteer potato include cultivation, hand-weed-ing, and herbicides (Boydston and Seymour 2002; Williamsand Boydston 2002); however, cultivation is limited to thecrop interrow. In addition, hand-weeding is most effective
when potatoes are large enough to retain daughter tubersduring weed removal. Bromoxynil and oxyfluorfen, whichsuppress volunteer potato, are labeled for use in onions thathave two or more fully emerged leaves (Boydston and Sey-mour 2002; Eberlein et al. 1993). As with hand-weeding,volunteer potatoes can interfere with onions for a relativelylong period before POST herbicides can be used.
Development of integrated weed management (IWM)systems for onion that target persistent weeds such as vol-unteer potato will require a greater understanding of whenweed interference can be tolerated before crop yield is re-duced. Critical timing of weed removal is defined as themaximum length of time early-season weed interference canbe tolerated by the crop before the crop becomes subjectedto yield reduction (Knezevic et al. 2002). Researchers havedetermined that the critical timing of weed removal beginsbetween two- and four-leaf stage of onion for small-seededannual weeds such as common lambsquarters (Chenopodiumalbum L.) and redroot pigweed (Amaranthus retroflexus L.)(Dunan et al. 1996; Hewson and Roberts 1971; Shadboltand Holm 1956). Weed densities were high (50 to 850plants m22) in these studies, relative to observed densitiesof volunteer potato (0.4 to 10 plants m22) in the PacificNorthwest (R. Thornton, personal communication). Morerecently, simulation modeling by Dunan et al. (1995, 1996)suggests weeds may need to be controlled before the two-leaf stage of onion to avoid yield reduction.
The overall goal of this study was to determine the sig-nificance of volunteer potato interference in current onion-production systems. Specific objectives were to (1) quantifythe influence of duration of volunteer potato interferenceon onion yield and market grade, (2) determine the criticaltime for volunteer potato removal, and (3) quantify the tem-poral dynamics of volunteer potato tuber production whengrown in onion.
Williams et al.: Duration of interference in onion • 63
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Materials and Methods
Irrigated field experiments were conducted at Parma, ID,Ontario, OR, and Prosser, WA, in 2002. Onions were plant-ed 2 cm deep using a seeder equipped with four plantingshoes spaced 56 cm apart. A planter with a single onion lineper planting shoe was used in Washington.1 Onions at Ida-ho and Oregon were planted on 28-cm-wide raised bedswith double lines, spaced 7.6 cm apart, per planting bedper shoe.2 Additional details on planting, emergence, andsite characteristics are shown in Table 1.
The experimental design was a randomized complete blockwith four replications per treatment and plots measured 2.2by 7.5 m. A single volunteer potato density of 2 plants m22
was established and maintained in each plot, and eight du-rations of interference treatments were established. Increasingdurations of weed interference were accomplished by delayingvolunteer potato removal time according to onion leaf num-ber and were designated as follows: removal at onion emer-gence (weed free) or at one-, two-, three-, four-, six-, or eight-leaf. In addition, a treatment was included in which volunteerpotato was allowed to grow for the entire season (no re-moval). Once the end of the interference duration wasreached and volunteer potato removed, the treatment waskept weed free for the rest of the season. Within 2 d of onionplanting, whole potato tubers, averaging 59 g tuber21, wereplanted to simulate volunteer potatoes. The exception wasIdaho, where onion was replanted because of seedling mor-tality from cool, wet soils (Table 1). Potato tubers were hand-planted 15 cm deep, either between the two onion lines (Ida-ho and Oregon) or within 2 cm of the single onion line(Washington). Tubers were not planted in outside rows ofexperimental units. At all sites, tubers were spaced equidis-tantly within the onion line(s). Interference duration wasended, depending on the treatment, by clipping potato shoots2-cm above the soil surface and brushing on a 5% solutionof glyphosate or fluroxypyr to shoot stumps. Treated potatoesthat produced new leaves were immediately retreated untilgrowth stopped.
Experiments were kept free of weeds, except for potatoes,by hand-weeding and POST applications of 6.7 kg DCPAha21 and 1 kg pendimethalin ha21 and two POST appli-cations of 0.2 kg sethoxydim ha21 at the one- and two-leafstage of onion. Onions were furrow (Idaho and Oregon) orsprinkler irrigated (Washington) and fertilized according tosoil tests and university recommendations (Pelter et al.1992). Lambda-cyhalothrin was applied in Oregon andWashington as needed to control thrips.
After crop senescence, onions from the center 6 m (Idahoand Washington) or entire length (Oregon) of each plotwere hand-harvested on October 8, September 15, and Sep-tember 9, 2002, at Idaho, Oregon, and Washington, re-spectively. On the basis of maximum onion bulb diameter,bulbs were sorted by market grades, including small (, 5.7cm, i.e., nonmarketable), medium (5.7 to , 7.6 cm), jumbo(7.6 to , 10.2 cm), colossal (10.2 to , 10.8 cm), and su-percolossal (5. 10.8 cm).3 The number of bulbs and massof each market grade were recorded, with the exception be-ing Oregon, where total bulb number was recorded. Wheneach duration of weed interference treatment was completedat Oregon and Washington, one volunteer potato plant perplot was removed, and oven-dry shoot biomass and daughter
64 • Weed Science 53, January–February 2005
tuber number were determined. Idaho was unable to collectvolunteer potato shoot biomass or tuber number data.
Statistical Analysis
Relative yield was calculated within each block as yieldat a given duration of weed interference treatment dividedby weed-free yield within that block and expressed as a per-centage. Growing degree days (GDD) accumulated after50% onion and potato emergence were obtained using abase temperature of 7.2 C (Dunan et al. 1996). Minimumand maximum daily temperatures were obtained from anautomated weather station located within 1 km of the ex-periments.
A logistic equation was used, as described by Knezevic etal. (2002), to describe the effect of increasing duration ofweed interference on relative yield:
Y 5 ((1/(exp(c(T 2 d )) 1 f )) 1 (( f 2 1)/f ))100 [1]
where Y is yield (% of season-long weed free), T is time(expressed in GDD from onion emergence), d is the pointof inflection (i.e., time to one-half of the season-long yieldloss expressed in GDD), and c and f are constants. Equation1 was fit to relative onion yield, and model parameter esti-mates were then used to determine the amount of time,expressed as GDD and related to onion growth stage, need-ed to result in predetermined levels of yield loss of 2.5, 5.0,and 10%.
Onion yield was related to shoot biomass, at the timeduration of interference treatments were completed, using arectangular hyperbolic equation (Cousens 1985):
I 3 N Y 5 Y 1 2 [2]wf I 3 N
100 3 1 1 1 2A
where Ywf is derived weed-free yield, N is shoot biomass(expressed in g m22), I is percent yield loss as weed biomassapproaches zero, and A is the upper asymptote or maximumyield loss.
Density of daughter tubers produced by volunteer potatoin onion was described as a function of GDD using a lo-gistic model:
aY 5 [3]
T 2 T01 1 exp 21 2b
where Y is tuber production, T is time (expressed in GDDfrom potato emergence), T0 is time resulting in 50% tuberproduction, and b is the slope.
All equations were fit to data for each location using aniterative least squares procedure (SigmaPlot 8.04). Lack offit was assessed by reporting standard errors of parameterestimates, plotting predicted and observed values, or calcu-lating R2 values, or all. The extra sum of squares principlefor nonlinear regression analysis (Ratkowsky 1983) was usedto evaluate the similarity of parameter estimates among lo-cations. Comparisons were made by calculating a varianceratio of individual and pooled residual sums of squares andperforming an F test described by Lindquist et al. (1996).If parameter estimates were constant across locations, data
were pooled among locations. The significance of all statis-tical tests was alpha 5 0.05.
Results and Discussion
Weed-free onion yields were 58,800, 113,900, and80,200 kg ha21 for Idaho, Oregon, and Washington, re-spectively (data not shown). Lower yields in Washingtoncompared with Oregon largely were attributed to lowerseeding rates because of the use of single line per row plant-ing equipment in Washington (Table 1). Lower yields inIdaho compared with Oregon were attributed to a late re-planting date, resulting in a shortened growth period. De-spite differences among locations, weed-free yields werecomparable with or exceeded average reported yields foreach state (Anonymous 2003).
Duration of volunteer potato interference had a signifi-cant effect on onion yield, with season-long weed interfer-ence resulting in greater than 80% relative yield loss (Figure1). The F test for comparing nonlinear models indicatedthat onion response to duration of potato interference wasnot consistent among locations; therefore, data are presentedseparately for each location. The GDD at which one-halfof the season-long yield loss occurred (d parameter 5 in-flection point) ranged from 487 at Idaho to 1,082 at Wash-ington. This level of yield reduction required fewer GDDat Idaho than at Oregon and Washington, largely due tothe fact that onion was at a competitive disadvantage atIdaho where the onion crop had to be replanted. Volunteerpotato emerged 9 and 16 d after onion emergence inOregon and Washington, respectively, whereas volunteer po-tato emerged 10 d before onion emergence in Idaho becauseof onion replanting. Relative crop–weed emergence at Idahowould be more representative of an early-emerging volunteerpotato population left unchecked until after crop emer-gence.
Relative onion yield was related to volunteer potato shootbiomass at the time of weed removal in Oregon and Wash-ington. The F test for comparing nonlinear models indicat-ed that onion response to potato interference, as measuredby volunteer potato shoot biomass at the time of weed re-moval, was consistent between locations; therefore, datawere pooled between these two locations (Figure 2). Relativeonion yield decreased hyperbolically as a function of vol-unteer potato shoot biomass. Others have used destructive(Harrison et al. 2001) or nondestructive (Bussler et al.1995) measures of weed biomass to develop predictive cropyield loss equations. However, in this study, a smaller per-centage of the variation in onion yield was explained byvolunteer potato biomass (R2 5 0.80) than by duration ofvolunteer potato interference (R2 5 0.83 to 0.92).
Yield and bulb number of each market grade was signif-icantly affected by duration of volunteer potato interference.Increasing duration of volunteer potato interference reducedonion bulb size, resulting in lower yields of most valuablemarket grades such as jumbo, colossal, and supercolossal(Figure 3). Consequently, as duration of volunteer potatointerference increased, bulbs in medium and small marketgrades accounted for a larger proportion of total onion yield.Although overall onion density was less affected by increas-ing duration of weed interference, reductions in bulb sizewith time resulted in a higher proportion of total bulbs
Williams et al.: Duration of interference in onion • 65
FIGURE 1. Relative onion yield as a function of duration of volunteer potato interference in Idaho (filled circles), Oregon (open circles), and Washington(open squares). Regression equations are: Idaho, relative yield 5 ((1/(exp(0.007(x 2 487)) 1 1.21)) 1 ((1.21 2 1)/1.21))100 (r2 5 0.83); Oregon, relativeyield 5 ((1/(exp(0.004(x 2 1,018)) 1 1.05))1((1.05 2 1)/1.05))100 (r2 5 0.90); and Washington, relative yield 5 ((1/(exp(0.002(x 2 1,082)) 1 1.04))1 ((1.04 2 1)/1.04))100 (r2 5 0.92). Growing degree day base temperature was 7.2 C.
FIGURE 2. Relative onion yield as a function of volunteer potato shoot biomass at the time weed removal treatments were initiated at Oregon and Washington(no potato shoot biomass data were collected at Idaho). Regression equation is relative onion yield 5 100[1 2 (0.325x)/(100(1 1 0.325x/100))] (r2 50.80).
being accounted for by small bulbs (Figure 4). For instance,although volunteer potato interference through the eight-leaf stage of onion resulted in a similar number of bulbs asthe weed-free treatment in Washington, 45% of the bulbswere small (nonmarketable) and the rest of the marketgrades accounted for only 30% of the weed-free yield. Thisstudy is consistent with that of Hewson and Roberts (1971)who reported that duration of annual weed interference de-creased onion yield primarily by decreasing bulb diameter.
The approach taken in these field studies provides a sta-tistical method of quantifying the beginning of critical pe-riod for weed control in onion, which begins early with
volunteer potato interference. As an example, 2.5% yieldloss occurred at 120 to 237 GDD, when onion had 0.6 to1.7 leaves per plant (Table 2). Five percent yield loss wasobserved when onion had 1.1 to 2.7 leaves, and 10% yieldloss occurred with as few as two leaves per plant at Idahoand 3.9 leaves per plant at Oregon. In contrast, several au-thors have found no yield loss when small-seeded, annualweed species such as prostrate knotweed (Polygonum avicu-lare L.) and redroot pigweed interfere until the two-leaf stageof onion (Hewson and Roberts 1971; Thomas and Wright1984) or longer (Menges and Tamez 1981; Shadbolt andHolm 1956). Our results indicate volunteer potato interfer-
66 • Weed Science 53, January–February 2005
FIGURE 3. Effect of duration of volunteer potato interference on onion yieldby market grade in Idaho, Oregon, and Washington. Market grades arebased on maximum onion bulb diameter: small (, 5.7 cm), medium (5.7to , 7.6 cm), jumbo (7.6 to , 10.2 cm), colossal (10.2 to , 10.8 cm),and supercolossal (5. 10.8 cm). Growing degree day base temperature was7.2 C.
FIGURE 4. Effect of duration of volunteer potato interference on oniondensity by market grade in Idaho, Oregon, and Washington. Market gradesare based on maximum onion bulb diameter: small (, 5.7 cm), medium(5.7 to , 7.6 cm), jumbo (7.6 to , 10.2 cm), colossal (10.2 to , 10.8cm), and supercolossal (5. 10.8 cm). Note that only total onion densityis reported for Oregon. Growing degree day base temperature was 7.2 C.
ence could cause 5% or more yield loss in onion by thetwo-leaf stage (Table 2). These results are more comparablewith those of Wicks et al. (1973), in that yield loss occurredwhen annual weeds were present during a relatively shortperiod of time from onion emergence to the two-leaf stageonion.
Volunteer potato produced a significant number ofdaughter tubers while growing in onion. The F test for com-paring nonlinear models indicated that volunteer potato tu-
ber production was consistent between Oregon and Wash-ington; therefore, data were pooled. Potato at our 2 m22
density allowed to remain in the plots for the entire seasonproduced an average of 42 daughter tubers m22 (Figure 5).Moreover, 50% of the tubers in this treatment were pro-duced within 1,028 GDD after potato emergence. Similarestimates of maximum volunteer potato tuber number havebeen quantified in both the presence of onion (Boydstonand Seymour 2002) and absence of a crop (Williams and
Williams et al.: Duration of interference in onion • 67
TABLE 2. Maximum amount of time (standard error in parentheses) early-season potato interference can be tolerated for three predeter-mined levels of onion yield loss. Onion leaf number related to each time is included.a
Location
Time for indicated yield loss
2.5%
GDDLeaf
number
5.0%
GDDLeaf
number
10%
GDDLeaf
number
ID 120 (82) 1.3 186 (96) 1.4 270 (96) 2.0OR 237 (129) 1.7 363 (127) 2.7 521 (111) 3.9WA 148 (114) 0.6 261 (142) 1.1 438 (133) 3.7
a Abbreviations: GDD, growing degree days; ID, Idaho; OR, Oregon; WA, Washington.
FIGURE 5. Volunteer potato tuber density in onion as a function of accumulated growing degree days after potato emergence for Oregon and Washington(tuber density data were not collected at Idaho). Regression equation is tuber density 5 41.8/(1 1 exp(2(x 2 1,027.9)/356.7)) (r2 5 0.90). Growingdegree day base temperature was 7.2 C.
Boydston 2002). These data indicate that only short periodsof time are necessary for volunteer potato growing in onionto produce large numbers of daughter tubers, which cansubsequently grow as volunteer potatoes in crops followingonion in the rotation. This volunteer potato problem couldpersist through time despite current control methods in-cluding POST herbicides, mechanical cultivation, or morerecently investigated methods such as arthropod herbivory.
Implications for IWM
Onion is subject to significant losses in yield and bulbdiameter when volunteer potato control is delayed untiltwo-leaf or greater onion stage. As observed in our study,crop yield loss of 5% or more can occur by the two-leafstage of onion, the earliest time when herbicides can beapplied for volunteer potato suppression. Potato interferencenot only reduces onion yields but also bulb size, which fur-ther reduces market value. Therefore, volunteer potato in-terference may have an even greater effect on total marketvalue than on total crop yield. Crop prices also should be afactor when considering timing of weed control in additionto the cost and efficacy of onion weed control (Dunan etal. 1995, 1999).
This study raises the importance of an IWM system in-
tegrating multiple weed-management tactics, including pre-ventative approaches to reduce volunteer potato populationdensities before onion planting. Improving tuber recoveryduring commercial potato harvest is perhaps the most ratio-nal solution. Soil fumigation, although primarily used tocontrol nematodes and other soil pathogens, has the poten-tial to greatly reduce the number of viable potato tubers(Boydston and Williams 2003). Using arthropod defoliationinflicted by the Colorado potato beetle [Leptinotarsa decem-lineata (Say)] has been proposed by Boydston and Seymour(2002) as an additional form of volunteer potato suppres-sion and shows promise when integrated with reduced her-bicide doses (Williams et al. 2004b). Moreover, the temporaldynamics of volunteer potato tuber production should beconsidered in developing IWM systems to minimize thenumber of viable daughter tubers remaining in the field afteronions are grown.
Sources of Materials
1 Planter, Singulaire 785, Stanhay Webb Ltd., Houghton Road,Grantham, Lincs, NG31 6JE, U.K.
2 Planters, Mel Beck Precision Planters, 214 Thunderegg Bou-levard, Nyssa, OR 97913.
68 • Weed Science 53, January–February 2005
3 Idaho-E, Oregon Onions, 118 North Second Street, P.O. Box909, Parma, ID 83660.
4 SigmaPlot 8.0, SigmaPlot 2002 for Windows, Version 8.02.SPSS Inc., 444 North Michigan Avenue, Chicago, IL 60611.
Acknowledgments
We appreciate the technical support of Virginia Prest andCharles Rice, as well as the comments of the Associate Editor andtwo anonymous reviewers.
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Received April 6, 2004, and approved August 31, 2004.
